Prototype writes on a per-evaluation class expression leak across evaluations and vanish from Object.keys (regression, train 268 / #11043 fix) #11134

Summary

On origin/main 36892b7 (merge train 268), a prototype property assigned on one evaluation of a function-body class expression is visible on instances of every other evaluation. The assigned key also no longer shows up in Object.keys(C.prototype) or hasOwnProperty. Node keeps each evaluation's prototype separate.

This breaks @redis/client's attachConfig, which does Class.prototype[name] = createCommand(...) once for the client class and once for its Multi class. The Multi command stubs become visible on the client, and the client commands on the Multi class.

Repro (no packages)

class A {} class B {}
function h(k: string, v: any) { const C = class { static s = 1; }; C.prototype[k] = v; return C; }
const X = h("x", 1);
console.log("A1", Object.keys(X.prototype), X.prototype.hasOwnProperty("x"), (X.prototype as any).x, Object.getOwnPropertyNames(X.prototype));
const Y = h("y", 2);
console.log("A2", (new Y() as any).x, (new X() as any).y, (new X() as any).x, Y.prototype.hasOwnProperty("x"));
function lit(v: any) { const C = class { static s = 1; }; (C.prototype as any).z = v; return C; }
const Z1 = lit(1), Z2 = lit(2);
console.log("B", (new Z1() as any).z, (new Z2() as any).z, Object.keys(Z1.prototype));
function nostat(k: string, v: any) { const C = class { #p = 1; }; C.prototype[k] = v; return C; }
const N1 = nostat("n1", 1), N2 = nostat("n2", 2);
console.log("C", (new N1() as any).n2, (new N2() as any).n1);

Node 26.5.1:

A1 [ 'x' ] true 1 [ 'constructor', 'x' ]
A2 undefined undefined 1 false
B 1 2 [ 'z' ]
C undefined undefined

Perry origin/main 36892b7:

A1 [] false 1 [ 'constructor' ]
A2 1 2 1 false
B 2 2 []
C 2 1

A heritage form (const C = class extends Base {}; C.prototype[k] = v) is also affected: Object.keys(C.prototype) is [] where Node gives [ 'qq' ].

Attribution

• The computed-key cases (A and C) are correct on the same tree with 34cbef7 ("reflect ClassBody accessors on per-evaluation class prototypes", mongodb: TypeError "Cannot read properties of undefined (reading 'default')" on connect #11043) reverted. That commit makes class_id_for_decl_prototype_object recognize a per-evaluation prototype as the template's declared prototype. Every reflection and write site keyed on that lookup then treats per-evaluation writes as template-wide.
• The literal-key case (B) goes through js_register_prototype_method(template_cid, …), confirmed with gdb. That side table is keyed by the shared template class id, so it is template-wide for any ClassExprFresh.

Found while rebasing PR #11122 (#11042) onto train 268. That PR's gap test test_gap_11042_class_expr_dynamic_heritage_per_evaluation.ts fails on origin/main because of this bug: typeof client.exec is function where Node gives undefined.